The present invention relates to recordable (once-writable) and rewritable information-recording media, and more particularly, to optical discs.
Recently, the densification of such optical discs, which are used as recording devices for computers, or package media for music and image information, has been advanced. In order to have precise, high-speed access to such high-density tracks, it is necessary to provide preformatted signals having address information to a recording surface of the optical disc, to and from which surface information is recorded and reproduced. Further, information that one does not want to erase can also be formed in advance as preformatted signals on the disc. Portions other than the preformatted portions are a recording area in which guide grooves referred to as simply xe2x80x98groovesxe2x80x99 are formed. In the recording area, grooves that are concave portions and lands that are not concave portions exist.
FIGS. 7A and 7B show a disc of related art, and are a top view and a perspective view of the disc, respectively. In FIG. 7B, a cross section of the disc is also shown. Reference symbols xe2x80x98Gxe2x80x99 and xe2x80x98Lxe2x80x99 indicate a groove and a land, respectively. Reference symbol xe2x80x98PPxe2x80x99 indicates a prepit. Light 3 is condensed by a lens 2 and then incident through a substrate 1. Comparing the land with the groove, the groove is nearer to the lens 2.
On the land, groove and prepit, a recording layer of a material represented by a magneto-optical material, a phase-change material or an organic dye material is formed (not shown in the figures). In the figures, a recording mark M is written in the groove. This is because better signals in quality are obtained when the information is written in the grooves than when written on the lands.
When recording/reproducing information onto/from the optical disc on which the guide grooves are formed as described above, tracking is implemented by a push-pull method. A light beam spot tracks on the grooves and prepits.
The shallower the depth of the groove, Dg, is, the larger signal amplitude of the marks written to the grooves is achieved, resulting in a good S/N ratio. Accordingly, as the discs become more densified, they are adjusted such that the depth of groove, Dg, is smaller, to thereby obtain signals with a good S/N ratio.
On the other hand, when Dp is xcex/4n, the amplitude of the prepit signal reaches its maximum. In the shallower prepit, the signal amplitude becomes smaller. Therefore, when both of Dg and Dp are adjusted to be small, or shallow, the prepit signals become too small, resulting in difficulty in reading the preformatted information that one does not want to erase, such as address information.
Also, when recorded on shallow prepits by mistake, since the amplitude of signals of written marks is larger than that of the original prepit signals, the information that has been originally preformatted as ROM information becomes unreadable. To make the matter worse, the ROM information goes rewritten, and thus a serious defect occurs in the reliability of the disc.
Furthermore, in the disc in which Dg=Dp, the magnitude of the push-pull signal in the prepit area is smaller than that of the push-pull signal in the groove area. Accordingly, if the gain of tracking is adjusted so that it is suitable for the groove area, the amplitude of the push-pull signals in the prepit portions becomes too small. Therefore, the control precision in the tracking servo deteriorates. In contrast, if the gain of tracking is adapted to the prepit area, the amplitude of the push-pull signal in the groove area becomes excessively large. Thus, disadvantageously, oscillation occurs in the tracking servo system.
In order to solve such problems, it is thinkable to switch the gain of tracking every time the beam spot reaches each of the groove and prepit areas. However, this will invite the complication of the circuitry of the recording/reproducing apparatus for the discs and thus may increase the costs of the apparatus.
The present invention has been made in order to solve the above problems, and the object of the present invention is to provide a highly reliable optical recording medium with prepits wherein recording is implemented at least at grooves, signals from the recorded marks and signals from the prepits have magnitudes large enough to obtain high reliability, and push-pull signals obtained in the groove and prepit portions have similar magnitudes so that precise tracking can be implemented, whereby complication of the circuitry of a recording/reproducing device and the increase of the costs of such a device can be avoided.
According to an aspect of the present invention, there is provided an optical recording medium, comprising a substrate having a recording area, composed at least of lands and grooves, and a prepit area in which information has been recorded beforehand in the form of prepits, said recording area being recordable at least at the grooves among said lands and grooves, said optical recording medium being constructed such that a depth of each groove, Dg, and a depth of each prepit, Dp, satisfy the following relationship:
Dg less than xcex/8n,
and
xcex/8n less than Dp less than xcex/4n,
respectively,
where xcex is a wavelength of light to be used for recording/reproducing of information signals, and n is an index of refraction of the substrate of the optical recording medium.
In the optical recording medium with the above arrangement, an RF signal having a large amplitude is obtained from both of the preformatted area (prepits) and the marks recorded at the grooves. At the same time, if overwrite occurs at the prepits, the overwrite has less influence on the reproduction of the prepit information, resulting in an improvement in the reliability of data.
In one embodiment, the depth of the grooves and the depth of the prepits have been adjusted so that a difference between a magnitude of a push-pull signal from the recording area and that of a push-pull signal from the prepit area is as small as possible.
With this arrangement, the amplitude of the push-pull signal from the recording area and that of the push-pull signal from the prepit area are made almost equal to each other. Thus, it becomes possible to achieve accurate tracking. Accordingly, there is an effect that it is possible to prevent the complication of the circuit and the increase in the cost of the recording/reproducing apparatus for the optical recording medium.
Preferably, the grooves and prepits may also satisfy the relationship of (xcex/8nxe2x88x92Dg) greater than (Dpxe2x88x92xcex/8n)
According to another aspect of the invention, there is provided an optical recording medium, comprising a substrate having a recording area, composed at least of lands and grooves, and a prepit area in which information has been recorded beforehand in the form of prepits, said recording area being recordable at least at the grooves among said lands and grooves, wherein the prepits having a depth different from a depth of the grooves are provided on an imaginary line extending from each of at least part of the grooves.
The depth of the prepits may be, preferably, larger than the depth of the grooves
Other objects, features and advantages of the present invention will be obvious from the following description.